1. Field of Industrial Application
The present invention relates to a plasma processing system, and more particularly, to a plasma processing system having an improved plasma source capable of supplying ions, electrons, neutral radicals and ultra-violet and visible light useful for a process of chemical vapor deposition (CVD) or etching micron-scale elements on integrated circuits in the semiconductor industry.
2. Discussion of Related Art
With the advance of 300 mm Si wafers (substrates) in the semiconductor industry, high density plasmas with uniform plasma density over the front surface of a substrate to be processed are greatly required. Even though the scale-up of existing plasma systems designed to process 200 mm wafers is one approach to meet the requirement, it is impeded by hardware difficulties of the existing plasma systems. Two such conventional plasma sources are illustrated in FIGS. 14 and 15, which are mainly used for the conventional 200 mm wafer plasma processing systems.
One example of the conventional plasma sources shown in FIG. 14, has a reactor 50 made of a metal, which is formed by a top plate 51, a bottom plate 52 and a cylindrical side wall 53. In the reactor 50, a substrate holder 54 on which a wafer or a substrate 61 is loaded is disposed at a lower position close to the bottom plate 52, and is parallel to both the top plate 51 and the bottom plate 52. The substrate holder 54 is electrically isolated from the reactor 50 by an insulator 57 and is supplied with a rf current generated by a rf electric power source 55 through a matching circuit 56 and a capacitor 60. The reactor 50 is electrically grounded through a wire 58. In accordance with the configuration of the reactor 50, a plasma is generated in the space 59 between the top plate 51 and the substrate holder 54 on the basis of capacitive coupling of rf electrical power.
FIG. 15 shows the other example of a conventional plasma source. In this example, the configuration of reactor 70 is almost the same as the reactor 50 shown in FIG. 14, except for an extra rf electrode 71. The reactor 70 also has the top plate 51, the bottom plate 52 and the cylindrical side wall 53, and it is made of a metal. Further, the reactor 70 is provided with the substrate holder 54 on which the substrate 61 is loaded, the rf electric power source 55, the matching circuit 56, the capacitor 60, the insulator 57 and the ground wire 58. The rf electrode 71 is placed slightly below the top plate 51 parallel to the substrate holder 54. The top rf electrode 71 is electrically isolated from the reactor 70 and is given a rf current by a rf electric power source 72 through a matching circuit 73. The rf current supplied to the rf electrode 71 usually has a frequency that is higher than that supplied to the substrate holder 54. The plasma is generated between the rf electrode 71 and the substrate holder 54 by the capacitive coupling of rf electrical power.
One of the major problems of the conventional plasma sources shown in FIGS. 14 and 15 is that the power transfer efficiencies from the rf electric sources (55, 72) to the plasma is low. This is due to the consumption of a considerable fraction of the applied rf power by unwanted ion acceleration. This is an inherent property of the capacitively coupled plasmas, and results in a lower plasma density. Further, since the 300 mm wafer processing is combined with the 0.25 m technology, it is considered that chemical processes must be carried out at a lower pressure, for example, about 10 mTorr. However, the plasma density of capacitively coupled plasmas further drops with the lowering of pressure. Thus, a higher process rate that is required for an economically viable system can not be obtained.
If the diameter of the substrate to be processed is small, for example, it is 200 mm, a higher rf electric power can be applied to increase the plasma density. If the diameter of the substrate to be processed is 300 mm, however, the applied rf power must be increased at least by 2.25 times in order to maintain the same power density because the surface area of the 300 mm wafer is 2.25 times larger than that of the 200 mm wafer. Therefore, the requirement for the rf electric power to maintain the desirable power density may limit some of applications.
In addition, when a 200 mm wafer processing system is scaled up to a 300 mm wafer processing system, the pumping speed in a processing chamber also must be increased in order to maintain the same reaction rates.
Owing to these hardware difficulties, the conventional plasma sources for a 200 mm wafer shown in FIGS. 14 an 15 can not be simply scaled up for 300 mm wafer plasma sources. In order to avoid these problems, it is important to design plasma sources that yield a higher plasma density over a 300 mm diameter region. Further, there must be a higher plasma uniformity over the surface of the 300 mm wafer because some semiconductor processing methods, such as a plasma assisted anisotrophic etching method, need a plasma uniformity more than 95% over the whole surface of the substrate to be processed.